I wouldn't say it's all that popular. Like
>>37268849says the Cynthia ship is way bigger. I can see why it exists though.
It's a villain ship that clicks a nice set of boxes: healing, corruption, age gaps, lolicon, angst/edge, and personality juxtaposition.
It can be appealing for both guys and girls since both Dawn (player avatar) and Cyrus (average 4channer) can function as self-insert characters.